1. Field of the Invention
The field of art to which the claimed invention pertains is the catalytic cracking of hydrocarbons. More specifically, the claimed invention relates to a method for the restoration of a used FCC catalytic composite.
2. Description of the Prior Art
There are a number of continuous cyclical processes employing fluidized solid techniques in which carbonaceous materials are deposited on the solids in the reaction zone and the solids are conveyed during the course of the cycle to another zone where carbon deposits are at least partially removed by combustion in an oxygen-containing medium. The solids from the latter zone are substantially withdrawn and reintroduced in whole or in part to the reaction zone.
One of the more important processes of this nature is the fluid catalytic cracking process in which heavy petroleum hydrocarbon feed stocks boiling in excess of about 400.degree. F. are converted to lower boiling hydrocarbons in the motor fuel boiling range by heating them in contact with an amorphous silica-alumina catalyst maintained in a fluidized state. While other composites comprising silica, e.g. silica-zirconia, silica-magnesia, etc., have been known to catalyze the cracking reaction, the silica-alumina composite has been by far the most widely accepted catalyst in the industry. More recently, improved catalysts having the capability of yielding greater proportions of high octane gasoline have been prepared by the inclusion of a finely divided zeolite, or crystalline aluminosilicate, either naturally occurring or synthetically prepared, within the amorphous silica-alumina matrix. Prior inventors have prepared, tested and compared hydrocarbon conversion catalysts comprising a finely divided crystalline aluminosilicate distributed in an amorphous silica matrix on the one hand, and in an amorphous silica-alumina matrix on the other hand. Examples of such catalysts are as described or claimed in U.S. Pat. Nos. 3,312,615; 3,392,110; 3,503,874; 3,592,778; 3,669,903; 3,696,023; 3,849,291; 3,926,778; 3,939,058; 4,001,106 and 4,100,219.
A common problem in the fluid catalytic cracking process is the gradual loss of the activity and selectively of useful products, i.e. gasoline, for production of the cracking catalyst due to irreversible changes in the molecular or crystalline structure of the components of the catalyst. The regeneration section of the typical fluid catalytic cracking (FCC) unit effects removal of carbon and hydrocarbonaceous deposits acquired by the catalyst in the reaction section, but is unable to preclude and in fact causes or contributes to the aforementioned irreversible changes. This problem of loss of activity and selectivity is known to the art and there are numerous inventions that attempt to deal with it, such as the invention of U.S. Pat. Nos. 3,926,778 and 3,939,668, both of which teach an expensive time consuming procedure comprising heat treatment of the spent catalyst at conditions more extreme than would be encountered in the typical regeneration section.
U.S. Pat. No. 3,770,615 teaches a method of increasing catalyst activity by adding a water-fluidizing molecular sieve (FMS) slurry directly to the hot catalyst inventory of an FCC unit. The FMS contemplated by this reference has a particle size range of from about 60 to 200 microns which is similar to the size range of the typical FCC catalyst particles. The method of this reference yields a physical mixture of separate particles of fresh zeolite with the aged particles of FCC catalyst. This mixture will have an improved catalytic activity as compared to the aged FCC catalyst particles alone, but the mixture can achieve no improvement in selectivity with regard to certain desired products.
We have discovered a method of treating a used FCC catalyst with an FMS that achieves not only an increase in activity, but also in selectivity.